Converting Christmas lights from battery to power

Forums › Home Workshop DIY › Converting Christmas lights from battery to power

FieldMouse

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#323646 28-Dec-2025 12:11

I have a small Christmas tree which has 4 strings of seed lights. Each of these run on 4.5v (a battery pack with 3 AA size, 1.5V batteries).
I use rechargeable batteries, but that still means that I have to recharge 12 batteries very 5 days or so. That's a pain, so I was looking to convert them to power.

I could cut the plastic battery packs back, so that only the switch part remains and I could connect each switch to a USB charger.
USB chargers are 5V and the difference of 4.5v to 5v should not cause an issue, but it means having 4 chargers.

I measured the current with a multimeter and the current varies. The lowest is about 40mA and the highest is about 300mA.
I calculate the total to be less than 1A, but if it was 1A, the wattage would be 5W. Basic USB chargers may not cater for 5W, but a lot of phone chargers do.

My question relates to the wiring. Can I connect the 4 switch parts in parallel? That would then mean, just one charger.

Thanks in advance

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#3447573 28-Dec-2025 12:14

Yes.

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gzt

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#3447577 28-Dec-2025 13:14

I recently did something similar. The lights are brighter, maybe too bright. It's been working off USB for six months 12/24 with no intervention needed.

FieldMouse

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#3447580 28-Dec-2025 14:06

Thank you

I found this - Dynamix SPAUSB-5V2A 10W Max Small Form USB Wall Charger

It is USB A, so can I use any USB A cable or does it need to be a charging cable?

FieldMouse

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#3447581 28-Dec-2025 14:06

Dynamic:

Yes.

Thank you

SCUBADOO

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#3447582 28-Dec-2025 14:07

Any normal USB cable will be fine.

A 50c 3A silicon diode in series with the 5V power supply positive or negative lead and the resulting c4.3V output may just enable the LEDs to last a very long time.

I have 5 inexpensive strings running here 24/7 that have all survived a few festive seasons.

FieldMouse

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#3447586 28-Dec-2025 14:51

SCUBADOO:

Any normal USB cable will be fine.

A 50c 3A silicon diode in series with the 5V power supply positive or negative lead and the resulting c4.3V output may just enable the LEDs to last a very long time.

I have 5 inexpensive strings running here 24/7 that have all survived a few festive seasons.

Thank you

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richms

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#3447599 28-Dec-2025 17:02

Put a 33 to 100 ohm resistor in series with it to be sure you are limiting the current to something sane.

Many crap battery light sets have a resistor in the battery case you can salvage out of it, otherwise it may be using the junk wire as a way to limit the current - that is more common on those LED fake neon signs.

Richard rich.ms

MarkM536

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#3447644 28-Dec-2025 21:41

richms:

Many crap battery light sets have a resistor in the battery case you can salvage out of it, otherwise it may be using the junk wire as a way to limit the current - that is more common on those LED fake neon signs.

^ this right here.

There's also some cheap battery powered lights that don't use a series resistor. Instead they rely on the batteries internal resistance, or cable's resistance. Eeek.

Nod

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#3447729 29-Dec-2025 10:53

If you add one diode in series, it will drop the voltage by 0.7V. 5.1-0.7=4.4V

Nod

FieldMouse

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#3448019 30-Dec-2025 10:18

Nod:

If you add one diode in series, it will drop the voltage by 0.7V. 5.1-0.7=4.4V

Nod

What diode do you suggest?
Note that I am keeping the switch mechanism because the lights have a 2 way switch (Steady/Flash) and the mechanism has a resistor.

richms

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#3448064 30-Dec-2025 10:20

1N4007 is the go to for that sort of load.

Richard rich.ms

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richms

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#3448065 30-Dec-2025 10:22

MarkM536:

There's also some cheap battery powered lights that don't use a series resistor. Instead they rely on the batteries internal resistance, or cable's resistance. Eeek.

I cooked up a fake neon that I got because I wanted to replace the long USB cable it came with and then I found that its crapness was a design intent. They put longer cables on the smaller signs for a reason.

Richard rich.ms

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#3448066 30-Dec-2025 10:25

richms:

1N4007 is the go to for that sort of load.

See message above
I am keeping the switch mechanism because the lights have a 2 way switch (Steady/Flash) and the mechanism has a resistor.

Will that make a difference?

richms

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#3448067 30-Dec-2025 10:28

No difference, The output will be PWM so the resistor is only relevant for what the max current that the LEDs take is, or a diode or 2 will be a cheap way of getting a somewhat constant lower voltage.

Where you run into issues with limiting current with resistors alone is if you had switched banks of LEDs, since the current would change. With all the normal LED controllers they are either on or off, no variance of what the load current is.

Richard rich.ms

FieldMouse

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#3448068 30-Dec-2025 10:34

richms:

No difference, The output will be PWM so the resistor is only relevant for what the max current that the LEDs take is, or a diode or 2 will be a cheap way of getting a somewhat constant lower voltage.

Where you run into issues with limiting current with resistors alone is if you had switched banks of LEDs, since the current would change. With all the normal LED controllers they are either on or off, no variance of what the load current is.

Thank you

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